Medical devices include devices designed to be implanted into a patient. Some examples of these implantable medical devices (IMDs) include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients or subjects using electrical or other therapy or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. Other examples of IMDs include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability.
Some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these events can include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs can detect abnormally slow heart rate, or bradycardia. In response to an abnormally slow heart rate some CFM devices deliver electrical pacing stimulation energy to induce cardiac depolarization and contraction (sometimes called capture of the heart). The stimulation energy is delivered to provide a depolarization rate that improves hemodynamic function of the patient.
Normally, a patient's heart rate changes in response to a change in physiologic need (e.g., exercise). However, some patients' ventricular depolarization rate may not adequately change in response to a change in physiologic demand. This condition is sometimes called chronotropic incompetence. The detection of chronotropic incompetence typically involves physician time with the patient in a clinic to conduct exertion tests to monitor a patient's response to exertion.
Examples of a cardiac device able to predict or recognize a chronotropically incompetent condition can be found in Scheiner et al., U.S. Pat. No. 7,142,920, “Chronotropic Status Monitor for Implantable Medical Device,” filed May 25, 2004, which is incorporated herein by reference in its entirety. Examples of calibration of adaptive rate pacing using an intrinsic chronotropic response can be found in Daum et al., U.S. Pat. No. 7,050,854, “Calibration of Adaptive-Rate Pacing Using Chronotropic Response,” filed Jun. 24, 2002, which is incorporated herein by reference in its entirety.